(1) Field of the Invention
The invention described herein relates generally to hydrophone signal processing and, more particularly, to systems and methods for calibrating optical hydrophone detector systems.
(2) Description of the Prior Art
A typical optical hydrophone has a reference leg and a sensing leg. The sensing leg is formed by wrapping a fiber optic cable around a compliant mandrel. The reference leg is formed by wrapping a length of fiber optic cable around a noncompliant mandrel. During operation, light is pulsed down both fiber legs and reflected by mirrors imbedded in the ends of the fibers. The output of both legs, the reference and sensing legs, are summed at a node forming an interferometer. This summation produces a phase modulating signal of the form
O=A+B cos xcex8(t)xe2x80x83xe2x80x83(1)
where
A and B=Constants proportional to the input power, and
xcex8(t)=Phase difference between the interferometer sensor and reference leg.
Typically, a sinusoidal modulating frequency is injected through a piezoelectric element on the reference leg of the interferometer. The output signal is given by
O=A+B cos(C cos xcfx89o(t)+x(t))xe2x80x83xe2x80x83(2)
where
x(t)=Signal of interest,
C=Modulating signal amplitude, and
xcfx89o=Modulating signal frequency.
Analog demodulators are used to process the output signal. These demodulators are complex custom-built hardware, requiring both expensive and time-consuming calibration. What is needed is an improved system for using programmable digital signal processor for demodulation and for calibration.
Patents that show attempts to solve the above and other related problems are as follows:
U.S. Pat. No. 4,977,546, issued Dec. 11, 1990, to Flatley et al., discloses a system for signal stabilizing in-phase modulated optical hydrophone arrays employing interferometry with homodyne detection. Phase stabilization is accomplished by modulating the input laser signal in proportion to variations in the output of an optical transducer to balance the output phase so that the fringes are kept at optimum position. Additionally, fluctuations in light intensity are compensated for so that a photodetector responds only to phase shift variations. The technique used is to split the input beam into signal and reference beams using a beam divider, exposing the signal beam to the acoustic pressure of interest, recombining the signal beam with the reference beam, detecting the combined beams and filtering the resulting signal to separate out the acoustic information of interest from the phase shift and light intensity portions used to stabilize the input beam. The acoustic information is processed and the phase shift and light intensity information provides a feedback signal for use in input beam stabilization.
U.S. Pat. No. 5,313,266, issued May 17, 1994, to Keolian et al., discloses a highly sensitive optical fiber interferometer sensor comprising a laser light source, a [2xc3x972] optical fiber coupler to split the beam in two, a differential transducer which converts a signal of interest into optical phase shift in the laser light transmitted through the two optical fibers in the interferometer and a [3xc3x973] optical fiber complex which recombines the two beams, producing interference which can be electronically detected. The use of the [3xc3x973] coupler permits Passive Homodyne demodulation of the phase-modulated signals provided by the interferometer without feedback control or modulation of the laser itself and without requiring the use of electronics within the interferometer.
U.S. Pat. No. 5,345,172, issued Sep. 6, 1994, to Taguchi et al., discloses a means to accomplish double-slice imaging by a nuclear magnetic resonance (NMR) imaging apparatus having an ordinary radio frequency magnetic field generator, two radio frequency magnetic field waveforms are used and slices are separated by subsequent calculation. More definitely, two slice portions are excited in a REAL direction by a COS waveform and are excited in an IMAG direction by a SIN waveform. When one of the slices is S1 with the other being S2, the signal SC when the COS waveform is used is S1+S2 while the signal SS when the SIN waveform is used is i.S1xe2x88x92i.S2. Therefore, the calculation for separating the slices proves SC+i.SS and SCxe2x88x92i.SS.
U.S. Pat. No. 5,809,087, issued Sep. 15, 1998, to Ashe et al., discloses an architecture for remote calibration of coherent systems using coherent reference and calibration signals that contain the relative amplitude and phase information desired in the calibration process. Circuitry extracts the relevant amplitude and phase information needed for the calibration while compensating for non-synchronized clocks and the effects of Doppler shifts due to relative motion of the transmitting and receiver platforms. The coherent detection architectures can be used effectively with any scheme designed to determine the relative amplitudes and phases of the signals emitted from the different elements of the phased array. These architectures are particularly applicable to coherent encoding calibration procedures that enhance the effective SNR by using coherent transmission of orthogonal transform encoded signals from N elements of the phased array. In an example calibration architecture, coherent elemental signals are encoded using controlled switching of the delay phase control circuits themselves to effectively generate a perfect orthogonal transform encoding of the signal vectors, even though the control circuits may be imperfect; no additional encoding hardware is required. The switching is dictated by matrix elements of an Nxc3x97N invertible binary matrix, with the most preferred embodiment being an orthogonal binary matrix, i.e., a Hadamard matrix. The coherent signals are decoded with the inverse of the same binary matrix used in the control circuit encoding.
U.S. Pat. No. 5,894,280, issued Apr. 13, 1999, to Ginetti et al., discloses a digital to analog converter (DAC) offset autocalibration system in a digital synthesizer integrated circuit. The present invention includes a DAC coupled to a filter. The input of the DAC accepts digital values for conversion to an analog signal. The output of the DAC is coupled to the input of the filter. The filter smoothes the analog signal received from the DAC. A switch is coupled to the filter output to receive the analog signal. A comparator is coupled to the switch. The input of the comparator receives the analog signal from the filter output via the switch. An autocalibration control circuit is coupled to the output of the comparator, to the switch, and to the DAC. The autocalibration control circuit is adapted to input a value to the DAC in order to determine an offset correction from the output of the comparator and adjust the analog signal using the offset correction.
U.S. Pat. No. 5,903,350, issued May 11, 1999, to Bush et al., discloses an apparatus and method providing wide dynamic range measurements of the input phase to an interferometer using a phase generated carrier. This invention is useful when utilizing time multiplexing to demodulate a series of interferometers. A modulation drive output is provided by the invention and maintained under operation at the optimum amplitude by an internal feedback loop. The resulting highly stable system can be fabricated from an analog to digital converter, a digital signal processor, and a digital to analog converter making low cost open loop demodulators a reality.
U.S. Pat. No. H1619, issued Dec. 3, 1996, to McCord et al., discloses a frequency modulated monitor hydrophone system which is used to monitor low frequency sound signals where cross-talk coupling is a problem. The invention consists of a hydrophone, preamplifier and receiver which includes a control group. The hydrophone comprises an acoustic sensor and low-noise preamplifier utilizing dynamic range compression to condition the electrical acoustic sensor signal before it is frequency modulated (FM) and applied to a coaxial cable. At the remotely located receiver, the FM signal from the hydrophone preamplifier is filtered to remove undesirable signals, such as audio spectrum crosstalk and out of band signals. The partially recovered audio signal is decompressed utilizing dynamic range decompression, amplified, and output for utilization or recordation by an operator. A calibration circuit provides a continuity or partial calibration check for the hydrophone by applying a signal of predetermined frequency and voltage to the hydrophone preamplifier and sensor. A microprocessor in the control group periodically reads the input signal and controls the various receiver and hydrophone preamplifier circuits. Selected controls on the panel of the control group allow the operator to set gains, perform the calibration procedures, and monitor system performance.
The above-cited prior art does not show a highly reliable means for accurately calibrating a digital optical hydrophone demodulator. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.
It is an object of the present invention to provide an improved calibration module for a demodulator which may be utilized with an optical hydrophone system.
It is another object of the present invention to provide a calibration module as aforesaid which is highly reliable for determining an accurate phase alignment between a carrier and a received signal.
It is a further object of the present invention to provide a calibration module as aforesaid which is completely automatic and may be utilized within a multisensor system comprising large numbers of hydrophones.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. It will be understood that above listed objects and advantages of the invention are intended only as an aid in understanding aspects of the invention, are not intended to limit the invention in any way, and do not form a comprehensive list of objects, features, and advantages.
In accordance with the present invention, a process is provided for calibrating an optical hydrophone demodulator by determining a phase for phase alignment between a carrier and a received signal. The optical hydrophone demodulator produces a first output and a second output such that the first output is in phase quadrature with respect to the second output. The process may comprise one or more steps such as, for instance, comparing the first output with respect to the second output by varying the phase until a plot of the first output with respect to the second output is a straight line, storing a value of the phase when the plot of the first output with respect to the second output is a straight line, and adjusting the value of the phase by a predetermined amount to produce an adjusted phase such that the received signal is in phase with the carrier.
Other steps may include providing the adjusted phase to a first mixer utilized for producing the first output and providing the adjusted phase to a second mixer utilized for producing the second output. In a preferred embodiment, the first mixer comprises a first mixer table and the second mixer comprises a second mixer table.
Additional steps of the invention may includes determining a ratio of a maximum of the first output with respect to the second output, adjusting the phase to reduce the ratio below a predetermined value, maintaining a count related to a number of adjustments to the phase, comparing the ratio before and after a step of adjusting the phase, utilizing the count and the step of comparing to determine when to make a series of fine adjustments to the phase.
The process also provides for utilizing the adjusted phase for determining a scaling factor for the first output and the second output.
In other words, a programmed process is provided for calibrating an optical hydrophone demodulator comprising one or more steps such as, for instance, determining a ratio of a maximum value of the first output with respect to a maximum value of the second output, reducing the ratio by making adjustments to the phase in steps until a minimum value of the ratio is determined, storing a value of the phase when the minimum value of the ratio is determined, and adjusting the value of the phase by a predetermined amount to produce an adjusted phase such that the received signal is in phase with the carrier.
Additional steps may include determining a scalar attribute by measuring the ratio with the adjusted phase, and utilizing the scalar attribute for adjusting an amplitude of the first output and the second output.
The method may also comprise providing the adjusted phase to a first mixer utilized for producing the first output, and providing the adjusted phase to a second mixer utilized for producing the second output.
The present invention provides a calibration processor operable for automatically calibrating the optical hydrophone demodulator by determining a phase for phase alignment between a carrier and a received signal wherein the processor comprises one or more elements such as, for instance, at least one offset adjustment for varying a phase offset, a counter for counting the number of times the at least one offset adjustment varies the phase offset, and an initializer for setting the counter at an initial value.
Other elements may preferably comprise a plurality of decision modules for making decisions regarding a ratio related to the first output and the second output. In a preferred embodiment, the counter and the offset adjustment are operative in response to at least one of the plurality of decision modules. The plurality of decision modules may comprise a first decision module for determining whether the ratio is less than a predetermined number, a second decision module for determining whether the ratio increases or decreases after an offset adjustment is made, and a third decision module for determining whether the counter has a count greater than a predetermined value.
Other elements may include a first offset adjustment for making a course phase offset adjustment, and a second offset adjustment for making a fine phase offset adjustment wherein the course phase offset adjustment changes the phase offset by a greater amount than the fine phase offset adjustment. A third phase-offset adjustment may be provided for making a predetermined offset adjustment in response to at least one of the plurality of decision modules to thereby determine a value for the phase.
A scalar determination module may be provided for determining a scalar value related to the first output and the second output wherein the scalar determination module utilizes the phase for determining the scalar value.